Programming WebLogic
Enterprise JavaBeans

Session EJBs

This section describes how session beans work within the EJB container, and provides design and development guidelines that are specific to session beans. For a description of the overall bean development process, see Implementing Enterprise Java Beans.

Pooling for Stateless Session EJBs

By default, no stateless session EJB instances exist in WebLogic Server at startup time. As individual beans are invoked, WebLogic Server initializes new instances of the EJB.

However, in a production environment, WebLogic Server can provide improved performance and throughput for stateless session EJBs by maintaining a free pool of unbound stateless session EJBs—instances that are not currently processing a method call. If an unbound instance is available to serve a request, response time improves, because the request does not have to wait for an instance to be created. The free pool improves performance by reusing objects and skipping container callbacks when it can.

Upon startup, WebLogic Server automatically creates and populates the free pool with the quantity of instances you specify in the bean’s initial-beans-in-free-pooldeployment element in the weblogic-ejb-jar.xml file. By default, initial-beans-in-free-pool is set to 0.

Additionally, in this release of WebLogic Server, Administrators can initialize EJB pools on demand via the Administration Console. When an EJB’s pool is initialized, it is reset to the state it was in immediately after the EJB was deployed. For more information, see Initialize the idle bean cache and pool of an EJB in Administration Console Online Help.

The following figure illustrates the WebLogic Server free pool, and the processes by which stateless EJBs enter and leave the pool. Dotted lines indicate the “state” of the EJB from the perspective of WebLogic Server.

If you configure a pool, WebLogic Server will service method calls with an EJB instance from the free pool, if one is available. The EJB remains active for the duration of the client’s method call. After the method completes, the EJB instance is returned to the free pool. Because WebLogic Server unbinds stateless session beans from clients after each method call, the actual bean class instance that a client uses may be different from invocation to invocation.

If all instances of an EJB class are active and max-beans-in-free-pool has been reached, new clients requesting the EJB class will be blocked until an active EJB completes a method call. If the transaction times out (or, for non-transactional calls, if five minutes elapse), WebLogic Server throws a RemoteException for a remote client or an EJBException for a local client.

Note:

The maximum size of the free pool is limited by the value of the max-beans-in-free-pool element, available memory, or the number of execute threads.

You can configure the WebLogic Server to remove bean instances that have remained unused in the pool for a period of time specified in idle-timeout-seconds element in the pool element. When beans have been in the pool unused for the amount of time you specify in idle-timeout-seconds, they are removed from the pool until the number of beans in the pool reaches the number specified in initial-beans-in-free-pool; the number of beans in the pool will never fall below the number specified in initial-beans-in-free-pool.

When an application requests a bean instance from the free pool, there are three possible outcomes:

An instance is available in the pool. WebLogic Server makes that instance available and your application proceeds with processing.

No instance is available in the pool, but the number of instances in use is less than max-beans-in-free-pool. WebLogic Server allocates a new bean instance and gives it to you.

No instances are available in the pool and the number of instances in use is already max-beans-in-free-pool. Your application must wait until either your transaction times out or a bean instance that already exists in the pool becomes available.

Caching and Passivating Stateful Session EJBs

WebLogic Server uses a cache of bean instances to improve the performance of stateful session EJBs. The cache stores active EJB instances in memory so that they are immediately available for client requests. The cache contains EJBs that are currently in use by a client and instances that were recently in use. Stateful session beans in cache are bound to a particular client.

The following figure illustrates the WebLogic Server cache, and the processes by which stateful EJBs enter and leave the cache.

Figure 5-2 Stateful Session EJB Life Cycle

Stateful Session EJB Creation

No stateful session EJB instances exist in WebLogic Server at startup. Before a client begins accessing a stateful session bean, it creates a new bean instance to use during its session with the bean. When the session is over the instance is destroyed. While the session is in progress, the instance is cached in memory.

Stateful Session EJB Passivation

Passivation is the process by which WebLogic Server removes an EJB instance from cache while preserving its state on disk. While passivated, EJBs are not in memory and are not immediately available for client requests, as they are when in the cache.

The EJB developer must ensure that a call to the ejbPassivate() method leaves a stateful session bean in a condition such that WebLogic Server can serialize its data and passivate the instance. During passivation, WebLogic Server attempts to serialize any fields that are not declared transient. This means that you must ensure that all non-transient fields represent serializable objects, such as the bean’s remote or home interface. EJB 2.1 specifies the field types that are allowed.

Controlling Passivation

The rules that govern the passivation of stateful session beans vary, based on the value of the beans cache-type element, which can be:

LRU—least recently used, or eager passivation.

NRU—not recently used, or as lazy passivation

The idle-timeout-seconds and max-beans-in-cache elements also affect passivation and removal behaviors, based on the value of cache-type.

Eager Passivation (LRU)

When you configure eager passivation for a stateful session bean by setting cache-type to LRU, the container passivates instances to disk:

As soon as an instance has been inactive for idle-timeout-seconds, regardless of the value of max-beans-in-cache.

The container removes a passivated instance from disk after it has been inactive for idle-timeout-seconds after passivation. This is referred to as a lazy remove.

Lazy Passivation (NRU)

When lazy passivation is configured by setting cache-type to NRU, the container avoids passivating beans, because of the associated systems overhead—pressure on the cache is the only event that causes passivation or eager removal of beans.

The container:

Removes a bean instance from cache when idle-timeout-seconds expires, and does not passivate it to disk. This is referred to as a eager remove. An eager remove ensures that an inactive instance does not consume memory or disk resources.

Passivates instances to disk when max-beans-in-cache is reached, even though idle-timeout-seconds has not expired.

Specifying the Persistent Store Directory for Passivated Beans

When a stateful session bean is passivated, its state is stored in a file system directory. Each server instance has its own directory for storing the state of passivated stateful session beans, known as the persistent store directory. The persistent store directory contains one subdirectory for each passivated bean.

The persistent store directory is created by default in the server instance directory, for example:

D:\releases\<version>\bea\user_domains\mydomain\myserver\pstore\

The path to the persistence store is:

RootDirectory\ServerName\persistent-store-dir

where:

RootDirectory—the directory where WebLogic Server runs.

RootDirectory can be specified at server startup with the -Dweblogic.RootDirectory property.

ServerName—the name of the server instance.

persistent-store-dir—the value of the of the persistent-store-dir element in the <stateful-session-descriptor> element of weblogic-ejb-jar.xml. If no value is specified for <persistent-store-dir>, the directory is named pstore by default.

The persistent store directory contains a subdirectory, named with a hash code, for each passivated bean. For example, the subdirectory for a passivated bean in the example above might be:

Configuring Concurrent Access to Stateful Session Beans

In accordance with the EJB 2.x specification, simultaneous access to a stateful session EJB results in a RemoteException. This access restriction on stateful session EJBs applies whether the EJB client is remote or internal to WebLogic Server. To override this restriction and configure a stateful session bean to allow concurrent calls, set the allow-concurrent-calls deployment element.

If multiple servlet classes access a stateful session EJB, each servlet thread (rather than each instance of the servlet class) must have its own session EJB instance. To prevent concurrent access, a JSP/servlet can use a stateful session bean in request scope.

Design Decisions for Session Beans

This section discusses some design decisions relevant to session beans.

Choosing Between Stateless and Stateful Beans

Stateless session beans are a good choice if your application does not need to maintain state for a particular client between business method calls. WebLogic Server is multi-threaded, servicing multiple clients simultaneously. With stateless session beans, the EJB container is free to use any available, pooled bean instance to service a client request, rather than reserving an instance for each client for the duration of a session. This results in greater resource utilization, scalability and throughput.

Stateless session beans are preferred for their light-weight implementation. They are a good choice if your application’s beans perform autonomous, distinct tasks without bean-to-bean interaction.

Stateful session beans are a good choice if you need to preserve the bean’s state for the duration of the session.

Choosing the Optimal Free Pool Setting for Stateless Session Beans

When you choose values for initial-beans-in-free-pool and max-beans-in-free-pool you must weigh memory consumption against slowing down your application. If the number of stateless session bean instances is too high, the free pool contains inactive instances that consume memory. If the number is too low, a client may not obtain an instance when it needs it. This leads to client threads blocking until an instance frees up, slowing down the application.

Usually max-beans-in-free-pool should be equal to the number of worker threads in the server instance, so that when a thread tries to do work an instance is available.

WebLogic-Specific Configurable Behaviors for Session Beans

Table 5-2 summarizes the deployment descriptor elements you set to configure the behavior of a stateless session bean and how the bean behaves if you do not configure the element. All of the elements listed are sub-elements of the stateless-session-descriptor element in weblogic-ejb-jar.xml.

Table 5-3 summarizes the deployment descriptor elements you set to configure the behavior of a stateful session bean and how the bean behaves if you do not configure the element. All of the elements listed are sub-elements of the stateful-session-descriptor element in weblogic-ejb-jar.xml.

Table 5-2 WebLogic-Specific Features for Stateless Session EJBs

To control

Set the following weblogic-ejb-jar.xml element(s)

Default behavior

The number of inactive instances of a stateless session bean that exist in WebLogic Server when it is started.